1. Field of the Invention
Embodiments of the present invention generally relate to methods and apparatus for performing acoustic sensing based on Distributed Acoustic Sensing (DAS) with increased acoustic sensitivity.
2. Description of the Related Art
Sensing of a wellbore, pipeline, or other conduit/tube (e.g., based on acoustic sensing) may be used to measure many important properties and conditions. For example, formation properties that may be important in producing or storing fluids in downhole reservoirs comprise pressure, temperature, porosity, permeability, density, mineral content, electrical conductivity, and bed thickness. Further, fluid properties, such as pressure, temperature, density, viscosity, chemical elements, and the content of oil, water, and/or gas, may also be important measurements. In addition, downhole-logging tools based on sonic well logging systems may be used to measure downhole properties such as formation porosity, location of bed boundaries and fluid interfaces, well casing condition, and behind casing cement location and bonding quality. Monitoring properties and conditions over time may have significant value.
FIG. 1 illustrates a schematic cross-sectional view of a wellbore 102, wherein a distributed acoustic sensing (DAS) system 110 may be used to perform acoustic sensing. A DAS system may be capable of producing the functional equivalent of 10's, 100's, or even 1000's of acoustic sensors. Properties of downhole formations surrounding or otherwise adjacent the wellbore 102 may be monitored over time based on the acoustic sensing. Further, hydrocarbon production may be controlled or reservoirs 108 may be managed based on the downhole formation properties sensed by in-well acoustic measurement methods using the DAS system 110.
The wellbore 102 may have a casing 104 disposed within, through which production tubing 106 may be deployed. The DAS system 110 may comprise an acoustic energy source and a DAS device. The acoustic energy source may emit acoustic signals downhole. An optical waveguide, such as an optical fiber, within the wellbore 102 may function as the DAS device, measuring disturbances in scattered light that may be propagated within the waveguide (e.g., within the core of an optical fiber). The disturbances in the scattered light may be due to the acoustic signals, wherein the acoustic signals may change the index of refraction of the waveguide or mechanically deform the waveguide such that the Rayleigh scattered signal changes.
Acoustic sensing based on DAS may use the Rayleigh backscatter property of the fiber's optical core and may spatially detect disturbances that are distributed along the fiber length. Such systems may rely on detecting phase changes brought about by changes in strain along the fiber's core. Externally generated acoustic disturbances may create very small strain changes to optical fibers. The acoustic disturbance may also be reduced or masked by a cable in which the fiber is deployed. In order to better detect changes in strain from acoustic disturbances, a fiber optic cable that has increased acoustic sensitivity is desired.